Deploy rearchitected MusicMouse backend + web frontend

The app grew a python-backend/ and a web/ frontend, so pi_musicmouse no
longer matches it. Rework the role:

- Install a pinned uv and build the venv with Python 3.13; Raspbian
  Bookworm's python3 is 3.11 and the backend uses PEP 695 syntax.
- pip install python-backend/ instead of a requirements.txt, and take
  the systemd unit straight from the checkout so it can't drift.
- Build web/ on the control machine (no npm on the Pi) and rsync
  web/dist over. Chain the checkout -> install -> build -> sync -> restart
  steps through handlers so a run with no repo change does nothing.
- Per-host config files (config-<host>.yml); the schema now differs
  between a host with a real mouse and a display-only one.
- Version to deploy is per host (musicmouse_version), no default.

Add pi_kiosk: autologin user running startx with Firefox in kiosk mode
and no window manager, for a Pi with a monitor attached. Enabled by
mediapi_has_monitor, defaulting off.

Turn musicdolphin into a display-only instance: serial and audio
simulated, driven entirely through the kiosk page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

roles/pi_musicmouse/files/config-musicmouse.yml

@@ -0,0 +1,49 @@
+---
+general:
+  library:
+    root: /media/musicmouse
+
+  serial_port: "/dev/ttyUSB0"
+  alsa_device: softvol_effects
+  mqtt:
+    user: musicmouse
+    password: KNLEFLZF94yA6Zhj141
+    server: homeassistant
+  web:
+    host: "0.0.0.0"
+    port: 8080
+    static_dir: /opt/musicmouse/web/dist
+
+  # The old hass_url/hass_token keys don't exist in the current schema - HA
+  # integration now lives under general.ha (url, token, devices, scenes; needs at
+  # least one device or scene) - see config.yml.example. Add it back once picked.
+  button_leds_brightness: 0.5
+  volume_increment: 5
+  min_volume: 23
+  max_volume: 70
+
+figures:
+  elefant:
+    id: "88041174e9"
+    colors: ["#ffff00", "#00c8ff", "#094b46", "#c20099"]
+  fuchs:
+    id: "8804ce7230"
+    colors: ["#F4D35E", "#F95738", "#F95738", "#083d77"]
+  eule:
+    id: "88040d71f0"
+    colors: ["#e5a200", "#f8e300", "w33", "w99"]
+  omnom:
+    id: "88043c6ede"
+    colors: ["#005102", "#fec800", "#005102", "#3bc405"]
+  eichhoernchen:
+    id: "88040b78ff"
+    colors: ["#ff0ada", "#4BC6B9", "#69045a", "#4BC6B9"]
+  hund:
+    id: "8804bc7444"
+    colors: ["#ffff00", "#00c8ff", "#094b46", "#c20099"]
+  hase:
+    id: "88044670ba"
+    colors: ["#ffff00", "#00c8ff", "#094b46", "#c20099"]
+  schneemann:
+    id: "88043f71c2"
+    colors: ["#ff0ada", "#4BC6B9", "#69045a", "#4BC6B9"]